The Effect of Maternal Fat Specialization on Infant Birthweight
In order to maximize the number of infants weighing in the birthweight range associated with the lowest amount of morbidity and mortality (2,500-4,000 grams) the complex interactions between the anthropometric, social, and environmental factors that influence birthweight need to be better understood. Currently, maternal gestational weight gain recommendations are based on body mass index (BMI). However, BMI is a measurement of total maternal adiposity and does not account for fat specialization during pregnancy. It is hypothesized that upper body maternal fat stores are used to fuel fetal growth during pregnancy whereas thigh fat stores are reserved for lactation. Therefore, it is expected that maternal pre-pregnancy upper body fat will correlate with infant birthweight and lower body fat stores will not. This NIH-funded prospective cohort study follows 1,206 mother-infant pairs from first prenatal visit to after delivery with the objective of investigating the impact of maternal height, pre-pregnancy fat distribution, and gestational weight gain on infant birthweight. When birthweight was regressed on traditional determinants of birthweight (gestational age, sex of the infant, parity of the mother, maternal ethnicity, and smoking status) and maternal height, maternal gestational weight gain, circumference of the forearm at the beginning of pregnancy, and subscapular skinfold measurement at the beginning of pregnancy, 0.5423 of the variance in infant birthweight was explained (p = 0 .0001). Thigh adiposity was not associated with infant birthweight (p = 0.9819) and did not add to the explained variance. These results indicate that upper body fat directly contributes to fetal growth during pregnancy, with infant birthweight being independent of pre-pregnancy lower body fat stores. This is consistent with the growing body of knowledge that lower body fat stores primarily are used for lactation.